An anaerobic digester is a device that promotes the decomposition or “digestion” of organic waste material into simple organics and gaseous biogas products. Biogas is formed by the activity of anaerobic bacteria. These bacteria occur naturally in organic environments where oxygen is limited. Biogas is a mixture of methane, carbon dioxide, hydrogen sulfide and other volatile organic compounds. It is highly corrosive due to the presence of hydrogen sulfide and water vapor. The methane gas is useful once the sulfide compounds are removed.
Anaerobic digestion in open storage vessels has historically been utilized in waste water management, especially in livestock production, to reduce or convert complex organic matter to a smaller volume. This method has proven to be economical by reducing the volume of waste handled and by volatilizing some metabolites into the atmosphere. Some disadvantages of the process include poor ability to keep the anaerobic digestion process in balance, resulting in the release of malodorous gases, and inefficient, incomplete conversion of digested organic matter into biogas. The poor quality of methane gas yielded makes the economic recovery of methane gas generally infeasible.
A passive anaerobic digester is composed of an enclosed vessel containing a slurry of organic waste material along with gas collection devices and supporting mechanical equipment. It is difficult in passive anaerobic digestion systems to consistently maintain a rate of methanogenesis that makes the recovery of the methane gas feasible. Accordingly, most passive systems flare or treat the gas collected and release it the atmosphere.
Fixed-film anaerobic digesters are those in which a media is added to the vessel to increase the surface area and give the microorganisms a surface to which they can attach. Different media have been used including plastic beads, PVC pipe, wood chips, corncobs and other media. Advantages of the fixed film vessels are high solids retention time and lower hydraulic retention time, creating a more efficient process than other designs and allowing for smaller overall vessel volume and accordingly a smaller footprint. While having satisfactory odor control characteristics, the contamination of the methane gas recovered limits its usefulness.